This is a pretty simple set up to allow your guests to have access to your network on a different subnet and not be able to access other users on the network and the devices on your LAN.

Here are the steps:

Add a virtual interface in the wireless section. This will be labeled as ath0.1

Check the advanced box and enable AP isolation and set Network Configuration to "unbridged" and set up your IP address scheme and subnet mask.

Then apply the settings

Next we are going to add a DHCP server to this virtual interface.

Go to Setup then Networking

Here we are going to go down to DHCPD and we are going to add another DHCP server.

When we add it we are going to assign it to the virtual interface we created. The default identifier for this virtual interface is ath0.1

When that is selected we are going to press apply

Now you should have a virtual interface with a separate DHCP server on another subnet. Now we are going to add a code to the command so that the wireless guests will have no access to the devices on the LAN.

BTW, your instructions are more recent, simple and direct. I do not understand why the DD-WRT wiki suggested creating bridge for a single interface/VAP/ath0.1. Seems unnessarily complicated even though it may be elegant.

Its done that way in the dd-wrt wiki because this guide is so simple that it lacks basic functionality aspects, the fact that you even had to include a reference link to the dd-wrt wiki shows that. And the fact that you had post code for the 'enable NAT on WAN for guest network' shows even more how incomplete this guide here is... The dd-wrt wiki maybe 'elegant' as you put it, but it will also give your guest network FULL functionality - whereas this guide won't give you the full functionality of a guest network.

A number of intermediate steps are missing from these instuctions. Setting up an "unbridged" virtual interface creates a WLAN that cannot be assigned to a bridge. The firewall rules specified also requite some examination.

Unfortunately, the original author's signature line "I no longer work for Buffalo and am not associated with the company in any way", may be dissuading readers from challenging this post because of his implied expertise.